1. Field of Invention
This invention pertains generally to the field of lens fabrication and more particularly to the removal of a lens from a lens retaining block.
2. Description of Prior Art
The fabrication of lenses includes processing steps to generate both the surfaces of the lenses so as to impart specific optical properties to the lens, and also to accomplish the peripheral alteration, or edging, of the lenses. The first step in altering a lens is typically the generation of a surface on a partially finished lens blank. The second step in processing the lens is normally the peripheral alteration of the shape of the surfaced lens. The lens blanks and surfaced lenses may be, for example, spherical, cylindrical, optical flats, aspherical, or of multiple focal lengths. Once the lenses have been finished they may be put to a variety of uses such as spectacle lenses, camera lenses, or lenses used in instrumentation.
Edging the lens to obtain a desired shape involves a series of steps. Typically the optical center and, optionally the cylinder axis, of the lens is located and marked on a face. In those instances when the lens to be edged contains an asymmetric surface it is necessary that the optical center and cylinder axis of the lens be located and marked. Next, the lens is attached to a lens block by some type of holding mechanism, such as an adhesive, so that the optical center, and optionally the cylinder axis, of the lens are aligned with the center point and cylinder axis of the block. The desired peripheral shape is then imparted to the lens. During edging the temperature of the lens rises. The lens is often exposed to the steady flow of a coolant in order to prevent the lens from cracking.
Some means must be provided to attach the lens blank to the edging block with a bond that will not fail during alteration yet is possible to break once alteration is complete. In practice, the lens may be removed from the block by a variety of methods. For example, the lens may be pried from the block. However, this method has the disadvantage that the lens is often chipped, scratched, or otherwise damaged by the act of prying. This method can be facilitated by immersing the lens and block in hot water for a short period of time. However, some plastic lens materials cannot withstand such temperatures.
Another method of lens removal is the use of a fluid that is forced against the surface of a blocking pad that is adhered to the lens, thereby reducing the force holding the pad to the lens or block. Approximately two atmospheres of fluid gauge pressure are sufficient to reduce the holding force such that the lens may be easily separated from the block. The fluid used to achieve removal should comprise a gas or liquid that is nontoxic and which will be inert with respect to the lens block, the blocking pad, and the lens. Representative examples of useful gases for pneumatic lens removal include air, nitrogen, carbon dioxide, helium, and fluorocarbon gases. Representative examples of useful liquids for hydraulic lens removal include water, hydraulic oils, mineral oils and fluorocarbon liquids.
Another method of lens removal employs a tab that is pulled in the direction of the plane of the blocking pad so as to cause a reduction in the thickness of the pad and a progressive disengagement of the pad from the interface between lens and block. Removal may also be accomplished by placing the combination of lens, blocking pad and block into a cavity of the mounting block and then rotating the lens and the block in opposite directions with respect to each other, thereby causing them to separate. A specially designed hand tool may also be provided to accomplish this same result. The tool is not as wide as the mounting block and facilitates removal by making it easier to grasp the edge of the lens.
The latter method of lens removal is disclosed in U.S. Pat. No. 3,962,833 entitled METHOD FOR THE ALTERATION OF A LENS AND AN ADHESIVE LENS BLOCKING PAD USED THEREIN, issued to Johnson on Jun. 15, 1976: The problem repeatedly grasp pliers or a similar tool to remove the lens. Some level of skill is required to perform the lens removal operation rapidly while avoiding damage to the lens. After a period of time in such an occupation, the operator is likely to suffer various forms of fatigue and injury including, for example, carpal tunnel syndrome.
A final issue to be faced when removing a lens from an edging block is the relatively recent development and use of hydrophobic and oleo phobic coatings, which cause the surface of coated lens to have a relatively low coefficient of friction. The relatively slick frictionless lens surface required the development of adhesive pads having relatively greater adhesion which caused the aforementioned methods of lens removal to be generally less effective. In particular, the conventional methods of lens deblocking resulted in crazing, pitting or other scarring and damage to the lens, or additional treatment step to remove adhesive residue from the lens.